This invention relates to the recovery of oil from subterranean reservoirs and, more particularly, to new and improved secondary recovery operations utilizing flood water including a novel viscosifier for mobility control.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only a minor portion of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. In the supplemental techniques, which are commonly referred to as secondary recovery operations although in fact they may be primary or tertiary in sequence of employment, fluid is introduced into the reservoir in order to displace the oil therein to a suitable production system through which the oil may be withdrawn to the surface of the earth. The displacing medium may be a gas, an aqueous liquid, such as fresh water or brine, an oil-miscible liquid such as butane, or a water and oil-miscible liquid such as alcohol. Generally, the most promising of the secondary recovery techniques involves the injection into the reservoir of an aqueous flooding medium, either alone or in combination with other fluids.
One difficulty which often is encountered in secondary recovery operations is the relatively poor sweep efficiency of the injected displacing liquid. That is, the displacing liquid exhibits a tendency to channel through certain portions of the reservoir and to bypass other portions. Such poor sweep efficiency is occasioned by differences between the viscosity of the injected displacing medium and the in situ reservoir oil and also by permeability variations within the reservoir. The reservoir may comprise a plurality of fairly well-defined zones of widely diverse permeabilities. The injected displacing fluid preferentially flows through the more permeable zones of the reservoir thus leading to premature breakthrough of the displacing fluid at the production well or wells.
Even when the reservoir exhibits a relatively uniform permeability throughout, a situation referred to as instability fingering may develop in those instances where the viscosity of the injected displacing fluid is significantly less than the viscosity of the in situ reservoir oil. In this situation, the less viscous displacing fluid tends to develop fingers or bulges which may be caused by points of minute heterogeneities in the reservoir. These fingers of displacing fluid tend to become extended in the direction of flow and travel at a faster rate than the remainder of the injected fluid, thus again resulting in premature breakthrough at the production system.
Various techniques have been proposed in order to improve the sweep efficiency of the injected displacing fluid and thus avoid premature breakthrough. For example, it has been proposed in waterflooding operations to add thickening agents to at least a portion of the aqueous flooding medium in order to increase the viscosity thereof. The viscosity of the flooding medium may be increased prior to its injection into the reservoir or alternatively the viscosity may be increased in situ in order to avoid a reduction in injectivity at the injection well. For example, in U.S. Pat. No. 3,208,518 to Patton, there is disclosed a waterflooding process wherein the visocity of the flooding medium is increased in situ through the use of high molecular weight polymers such as ionic polysaccharides produced by the fermentation of carbohydrates by bacteria of the genus Xanthomonas, under controlled pH conditions.
Another technique whereby the viscosity of the aqueous displacing medium may be increased in situ involves the injection of a shear-thinning liquid. For example, as disclosed in U.S. Pat. No. 3,292,696 to Sandiford, an aqueous solution of hydroxyethyl cellulose which exhibits a relatively low viscosity at high shear rates may be injected in order to displace oil from the formation. Also, as disclosed for example in U.S. Pat. No. 3,315,743 to Abdo et al., it has been proposed to inject an aqueous shear-thickening liquid in a waterflooding procedure.
The concept of heating hydroxyethyl cellulose to increase its viscosity in aqueous solution is disclosed in U.S. Pat. No. 3,106,551 -- Lindenfors. Further, U.S. Pat. No. 3,677,961 -- Browning et al. discloses the crosslinking of a blend of a galactomannan gum and an ionic polysaccharide produced by the fermentation of carbohydrates by bacteria of the genus Xanthomonas with a chelate cross-linking agent and a metal ion as the acceptor selected from the class consisting of the metals of the first series of the transition elements of the Periodic Table and an organic polyelectrolyte ligand as the donor. The crosslinking of the hydroxyethyl cellulose with the use of polyvalent metal ions is also disclosed in U.S. Pat. No. 3,727,687 -- Clampitt whereas the preparation of viscous liquids by crosslinking solid water-soluble polysaccharides containing cis-hydroxyl groups with a cross-linking agent yielding borate ions in solution is disclosed in U.S. Pat. No. 3,215,634 -- Walker. The use of a chelating composition formed of an organic polyelectrolyte ligand component and a component of a metal of the first series of the transition elements of the Periodic Table for use in crosslinking macromolecular polysaccharide materials is disclosed in U.S. Pat. No. 3,697,498 -- Browning et al. Finally, the production of polyurethane prepolymers useful in the production of rigid foams by reacting an excess of a polyisocyanate and the addition product of an alkylene oxide having at least three carbon atoms with a hydroxy compound having at least six hydroxyl groups, such as a mixture of sorbitol and sucrose, is disclosed in U.S. Pat. No. 3,847,347 -- Satterly.